Tuesday, March 29, 2011

graceful degradation: Degradation of a system in such a manner that it continues to operate, but provides a reduced level of service rather than failing completely.

Graceful degradation is the ability of a computer, machine, electronic system or network to maintain limited functionality even when a large portion of it has been destroyed or rendered inoperative. The purpose of graceful degradation is to prevent catastrophic failure. Ideally, even the simultaneous loss of multiple components does not cause downtime in a system with this feature. In graceful degradation, the operating efficiency or speed declines gradually as an increasing number of components fail.

Graceful degradation has been an important consideration in the design and implementation of large communications networks since the Internet was originally conceived by the Advanced Research Projects Agency (ARPA) of the U.S. government. The Internet arose out of a desire on the part of public-service and military officials for a large-scale computer network that could resist massive physical as well as electronic attacks including global nuclear war. Today's Internet continues to maintain this resiliency although not to the extent all engineers would like. Malicious electronic activities in the form of viruses, worms, denial of service attacks and proliferation of spam cause considerable disruption from time to time.

Graceful degradation is sometimes considered equivalent to fault tolerance but there is a significant difference. Fault-tolerant systems are designed so that if a component fails or a network route becomes unusable, a backup component, procedure or route can immediately take its place with no negative impact whatsoever on individual subscribers. Graceful degradation is an outgrowth of effective fault management, which is the component of network management concerned with detecting, isolating and resolving problems.

In Web site design, the term graceful degradation refers to the judicious implementation of new or sophisticated features to ensure that most Internet users can effectively interact with pages on the site. Significant milestones in Web site design and Internet use over the years have included the introduction of images, frames, online gaming, Java, JavaScript, ActiveX controls, tabbed browsing, voice over Internet Protocol (VoIP) and videoconference technology. When an updated version of a browser or operating system is released, new features are often included to keep pace with the latest enhancements to the Internet. For various reasons, many Internet users prefer to continue using their existing browsers rather than immediately upgrading to the latest version every time a new Web site technology becomes popular. When a site is designed with graceful degradation in mind, such users are not abruptly forced to upgrade their browsers unless they are using "ancient" ones!

Tightly Coupled Systems

Multi Processor Systems also known as Parallel or Tightly Coupled Systems. Those systems that contain only one processor are known as single processor system. Multi processor systems have more than one processor that's why known as multi processor systems.

Some main advantages of multi processor systems are given below.

(1) As we increase the number of processors then it means that more work can be done in less time. Id more than one processor cooperates on a task then they will take less time to complete it.

(2) If we divide functions among several processors, then if one processor fails then it will not affect the system or we can say it will not halt the system, but it will affect on the work speed. Suppose I have five processors and one of them fails due to some reasons then each of the remaining four processors will share the work of failed processor. So it means that system will not fail but definitely failed processor will affect on its speed.

(3) If you pay attention on the matter of which save much money among multi processor systems and multiple single-processor systems then you will know that multiprocessor systems save more moneythan multiple single-processor systems. Because they can sharepower supplies, memory and peripherals.